Acyl-CoA:diacylglycerol acyltransferase (DGAT; EC 2.3.1.20) is a microsomal enzyme that plays a central role in the metabolism of cellular diacylglycerol lipids. It catalyzes the terminal and only committed step in triacylglycerol synthesis by using diacylglycerol (DAG) and fatty acyl CoA as substrates. MGAT uses mono-acylglycerol (MAG) and fatty acyl CoA as substrates. DGAT had been considered necessary for adipose tissue formation and essential for survival [Cases et al. (1998)].
Oelkers et al. [Oelkers et al. (1998)] identified 2 novel and distinct partial human cDNAs by using the sequence of human acyl-CoA:cholesterol acyltransferase-1 (ACAT1) to screen EST databases: They isolated two ‘ACAT-related gene products’ from a hepatocyte cDNA library which they named ARGP1 and ARGP2. ARGP1 was found to be expressed in numerous human adult tissues and tissue culture cell lines, whereas the expression of ARGP2 was more restricted. The ARGP1 cDNA encodes a protein of 488 amino acids with 9 predicted transmembrane domains, a potential N-linked glycosylation site, and a putative tyrosine phosphorylation motif. Comparison to ACAT1 revealed 22% amino acid identity over the entire molecule. Northern blot analysis of ARGP1 indicated ubiquitous expression with a great variability in level of expression. There was high expression in the adrenal cortex, adrenal medulla, testes, and small intestine, with moderate expression in thyroid, stomach, heart, skeletal muscle, and liver. A 2.0-kb transcript was invariable in all tissues examined, while a 2.4-kb transcript was observed in about half the tissues.
Later, Cheng et al. [Cheng et al. (2001)] cloned DGAT1 from an adipose tissue cDNA library and identified a splice variant, which they designated DGATsv. This DGATsv contains a 77-nucleotide insert of unspliced intron with an in-frame stop codon. It is a truncated form of DGAT1 that terminates at Arg387. Thereby 101 residues from the C-terminus are deleted, including the putative active site. By gel filtration, coimmunoprecipitation, and SDS-PAGE of cross-linked recombinant proteins, the authors determined that both DGAT1 and DGATsv form dimers and tetramers. When coexpressed, the 2 variants formed heterocomplexes.
Smith and co-workers [Smith et al., (2000)] demonstrated that DGAT-deficient mice which were generated by targeted disruption were viable and still synthesized triglycerides. Moreover they found that these mice were lean and resistant to diet-induced obesity. The obesity resistance involved increased energy expenditure and increased activity. DGATt deficiency also altered triglyceride metabolism in other tissues. This includes the mammary gland, where lactation was defective in DGAT −/− females. Smith et al. [Smith et al., (2000)] concluded that multiple mechanisms exist for triglyceride synthesis and suggested that the selective inhibition of DGAT-mediated triglyceride synthesis may be useful for treating obesity.
Buhman et al. [Buhman et al., (2002)] analyzed the DGAT1-deficient mouse model and found that DGAT1 was not essential for quantitative dietary triacylglycerol absorption, even in mice fed a high-fat diet, or for the synthesis of chylomicrons. However, DGAT1 null mice had reduced postabsorptive chylomicronemia 1 hour after a high-fat challenge. When chronically fed a high-fat diet, they accumulated neutral lipid droplets in the cytoplasm of enterocytes, suggesting reduced triacylglycerol absorption. Analysis of intestine from DGAT1 null mice revealed that the activity of enzymes involved in triacylglycerol synthesis, DGAT2 and diacylglycerol transacylase, may help to compensate for the absence of DGAT1. Using the positional candidate approach, Grisart et al. [Grisart et al. (2002)] mapped a quantitative trait locus with a major effect on milk composition in dairy cattle to the centromeric end of bovine chromosome 14, where the DGAT1 gene maps. They identified a nonconservative Lys232 to Ala substitution in the DGAT1 gene that had a major effect on milk yield and characteristics, including fat content.